It was not morally wrong for the lawyers to take the lead in building new tombs to replace the older tombs of Israel"s prophets. However, Jesus saw in this practice an ironic testimony to their opposition to God"s recent prophets, specifically John the Baptist and Himself. By building these tombs the lawyers appeared to be honoring the prophets, but they were also walling them in and sealing them off from the people. That was really what they were doing when they turned the people away from the prophets whom God had recently sent to Israel. In this they were following in the footsteps of their ancestors who killed the prophets. [source][source][source]
The relatives of a guilty criminal have sometimes given money to the family members of the victim of the criminal"s crimes, blood money to atone for their shared guilt. Perhaps the lawyers were building the prophets" tombs with the same motivation. [1]," Studia Evangelica4 (1968):187-93.] [source]